It is necessary to periodically measure the operating characteristics of airborne RF transceiving systems (e.g., secondary radar transponders) to facilitate adjustments and to ensure that they are operating properly. Unfortunately, it is typically time consuming, difficult and complicated to remove such a system from the aircraft for testing purposes--and in any event it is desirable to test the entire system as installed on the aircraft (including the antenna and associated RF cabling and connections). Accordingly, for these and other reasons it is generally desirable or necessary to make measurements such as receiver sensitivity and transmitted power with the system in place on the aircraft.
In the past it has been difficult to accurately measure such parameters of avionics RF transceiving systems while the systems are installed on the aircraft. Extraneous objects (e.g., wings and other structures on and off the aircraft) make such "in place" measurements difficult and inaccurate. For example, measurement difficulties arise due to RF field perturbations in the vicinity of the airborne antenna. Pick-up antennas have in the past been positioned within a few wavelengths of the airborne antenna of the system under test in order to couple RF power to and from the system. However, measurements made in this way are subject to errors due to near-field effects and standing waves caused by other objects within a few wavelengths of the pick-up antenna and/or the airborne antenna.
To alleviate this problem, pick-up antennas have sometimes been placed far away (e.g., thirty or more wavelengths) from the airborne antenna. At these distances, however, the reduced power levels of the received signals become troublesome. In addition, it is more difficult to assure at such large distances that the pickup antenna is located a predetermined distance from the airborne antenna (the accuracy in positioning the pickup antenna directly determines the accuracy of the resulting measurements in most cases).
Much prior work and analysis has been done in regard to the use of near-field measurements of avionics type and other RF radiators for ascertaining various antenna characterstics (e.g., radiation patterns). The following is a non-exhaustive listing of prior publications and patents relating to some such prior work:
U.S. Pat. No. 4,468,669 to Wang et al; PA1 U.S. Pat. No. 3,760,271 to Bach, Jr. et al; PA1 U.S. Pat. No. 3,641,439 to Asian; PA1 U.S. Pat. No. 4,588,993 to Babij et al; PA1 U.S. Pat. No. 4,704,614 to Poirier et al; PA1 U.S. Pat. No. 4,453,164 to Patton; PA1 U.S. Pat. No. 5,001,494 to Dorman et al; PA1 Kunath et al, "Near-Field Antenna Testing Using the Hewlett Packard 8510 Automated Network Analyzer", Paper, Antenna Measurement Techniques Assoc. Meeting, Philadelphia, Pa., NASA, 9 pp, (8-11 October 1990); PA1 R. I. Gray, "Measurement of Antenna Near-Fields", technical memorandum no. W-7/67, AD849027, Weapons Development Evaluation Lab., U.S. Naval Weapons Lab., Dahlgren Va., 7 pp, (March 1967); and PA1 Hanfling, "Planar Near-Field Measurements for Aircraft Antenna Applications", Raytheon Co., Bedford, Mass., 1979 Antenna Applications Symposium, Univ. of Illinois and Electromagnetic Sciences Div. of RADC, Hanscom, AFB at Allerton Park, Monticello, Ill., (26-28 September 1979).
The Hanfling et al. reference discusses, beginning at page 6, the difficulties of far field testing of installed airborne RF systems, and discloses a complicated technique using inverse Fourier transformations to measure the radiation pattern of a fuselage mounted broadbeam avonics antenna. However, it appears that a complicated x-y-z type rotating arm sensor shown in Hanfling's FIG. 12 would be required to obtain the data needed to perform such an analysis.
Wang et al. teaches a twin lead transmission line RF source for making near-field sensitivity measurements of a phased array type radar antenna.
Gray teaches a circular loop probe for measuring the levels of E and H fields in the near-field to provide measurements relating to hazards of electromagnetic radiation to ordnance.
Kunath et al. teach near-field testing of a large microwave radar antenna using an automated network analyzer connected to a probe (see FIG. 1).
Bach, Jr. et al. and Asian teach various non-antenna type probes for measuring power density in the near-field. However, such probes (e.g., diode arrays and thermocouples) would not be useful for measuring RF receiver sensitivity and have other problems as well.
Babij et al., Poirier et al., Patton and Dorman et al. teach additional complicated arrangements for mapping the radiation pattern of antennas based on near-field measurements.
There may be representative teachings in the prior art in addition to the references cited above. For example, it is generally known to place a small sensing antenna or probe a small distance from a radiating antenna for determining, for example, whether or not the radiating antenna is radiating a signal and for monitoring such radiated signal. Additionally, a manufacturer of military antennas several years ago advertised an antenna assembly including a probe disposed integrally within the antenna assembly itself. The advertised antenna assembly had two connectors: one for coupling RF energy to/from the antenna; and the other for connection to the probe. The probe may have been used for monitoring characteristics relating to the radiated output power of the antenna.
While much work has been done in the past relating to near-field measurements of RF transceiving systems, certain major problems have not been solved. For example, near-field measurements in the past have been costly and complicated. No simple, inexpensive device or system has been readily available in the past to provide accurate near-field measurements of certain parameters (e.g., radiated power level; and RF receiver sensitivity) of great importance to the testing of installed avionics RF systems. In addition, prior near-field measuring techniques required great attention and skill in order to provide accurate results. It would be highly desirable to provide a measuring technique that could provide accurate, repeatable results when performed by relatively unskilled technicians.
The present invention solves these and other problems inherent in prior art approaches by providing a low-cost RF probe and associated system that can be used for quickly, easily and relatively accurately measuring the transmitter radiated power and receiver sensitivity of an RF transceiving system (e.g., of the type mounted on the fuselage of an aircraft) while avoiding the standing wave and other troublesome effects encountered using past near-field antenna testing techniques.
In accordance with one aspect of the present invention, a coupling device is provided that makes use of the very close near-field at a distance so small that standing waves and the perturbations of nearby objects have very little effect on the measurements. For example, the near-field is used for measuring the sensitivity of the system by placing the signal injection so close to the antenna of the system under test that disturbances from objects in the far and intermediate fields have very little effect on the coefficient of coupling.
In accordance with another aspect of the present invention, the probe is placed so close to the antenna under test (e.g., 0.1 wavelengths away) that substantially only the tangential E field (and substantially no H field or radial E field) is coupled between the probe and the antenna. Measurements are simplified by the ability to ignore the effects of the H field and the radial E field.
In accordance with yet another feature provided by the present invention, a probe assembly is provided which is capable of mating with an antenna structure (e.g., the airborne blade type antenna of a secondary radar transponder antenna of the type that may be found on the fuselage of some aircraft). Such mating is providing by structure which conformally envelopes at least a portion of the antenna. By so mating the probe assembly with the antenna under test, a predetermined near-field spacing and alignment is accurately provided between a short RF pickup element contained within the probe assembly and the antenna under test. Measurements can thus be performed rapidly and accurately by a relatively unskilled technician--avoiding the need for complicated calibration procedures and equipment.